1. Field of the Invention
The present invention relates to an electron emission device, and in particular, to an electron emission device which has cathode and gate electrodes for controlling the emission of electrons from electron emission regions, and an anode electrode for accelerating the electrons.
2. Description of Related Art
Generally, electron emission devices are classified into a first type where a hot cathode is used as an electron emission source and a second type where a cold cathode is used as the electron emission source.
The second type of electron emission device may be a field emitter array (FEA) type, a surface-conduction emission (SCE) type, a metal-insulator-metal (MIM) type, or a metal-insulator-semiconductor (MIS) type.
The FEA-type electron emission device is based on the principle that when a material having a low work function or a high aspect ratio is used as an electron emission source, electrons are easily emitted from the electron emission source when an electric field is applied thereto under a vacuum atmosphere. A front sharp-pointed tip structure based on molybdenum (Mo) or silicon (Si), or a carbonaceous material such as graphite, has been applied for use as the electron emission source.
In common FEA-type electron emission devices, a first substrate and a second substrate make up a vacuum container. Electron emission regions are formed on the first substrate together with cathode and gate electrodes functioning as the driving electrodes for controlling the electron emission. Phosphor layers are formed on a surface of the second substrate facing the first substrate together with an anode electrode for keeping the phosphor layers in a high potential state.
The cathode electrodes are electrically connected to the electron emission regions to apply thereto the electric current required for electron emission, and the gate electrodes form electric fields around the electron emission regions using the voltage difference thereof from the cathode electrodes. In relation to the structure of the cathode and gate electrodes and the electron emission regions, the gate electrodes are placed over the cathode electrodes while interposing an insulating layer, and openings are formed at the gate electrodes and the insulating layer partially exposing the surface of the cathode electrodes. The electron emission regions are placed on the cathode electrodes within the openings.
With the above structure, predetermined voltages are applied to the cathode, gate, and anode electrodes to emit electrons from the electron emission regions. The electrons can be straightly migrated toward the second substrate without spreading only when an even potential distribution is made around the gate electrodes over the electron emission regions.
The even potential distribution means that, when viewing a side elevation view of the cathode and gate electrodes and the electron emission regions, the equipotential lines existing between the cathode and gate electrodes are located parallel to the top surface of the first substrate while being evenly spaced apart from each other by a predetermined distance. Equipotential lines not satisfying such a condition are considerably convex or concave in any one direction, so even potential distribution is not realized.
According to the operation principle of a known electronic lens, when the electrons pass through the interior of the electric field, the direction of electron migration is determined by the vector composition of the direction of electron migration and the direction of force (opposite to the direction of the electric field). In this regard, when a concave potential distribution directed toward the electron emission regions is formed around the gate electrodes, the electrons are considerably spread while passing through the openings of the gate electrodes. When a convex potential distribution directed toward the electron emission regions is formed around the gate electrodes, the electrons are focused while passing through the openings of the gate electrodes. However, the electrons are soon over-focused on the subsequent migration route, so that beam spreading also significantly occurs.
Accordingly, with the common FEA-type electron emission device, the potential distribution around the gate electrodes should be made as even as possible.
However, a considerable technical difficulty is encountered in making the even potential distribution because the potential distribution depends upon various factors, such as the voltages applied to the cathode, gate and anode electrodes, and the shape characteristic of the interior structure. As those factors also largely depend upon the discharge current characteristic of the electron emission regions, the screen brightness, and the processing capacity. There are technical limitations to optimizing the respective factors and to obtain the even potential distribution.
Consequently, with the conventional FEA-type electron emission device, a non-even potential distribution, that is, a convex or concave potential distribution directed toward the electron emission regions, is made around the gate electrodes during the operation thereof. The electrons emitted from the electron emission regions are spread while proceeding toward the second substrate, and land on black layers or incorrect phosphor layers, thereby deteriorating the screen display quality.